For producing gypsum, various calcining methods are known. During the calcining, wet gypsum (natural gypsum and/or gypsum from flue gas desulfurization installations) is converted by extracting water of crystallization. In chemical terms, this means the conversion of dihydrate (CaSO4*2H2O) to hemihydrate (CaSO4*½H2O) by means of heat. In this case, the alpha or beta modification of the hemihydrate can be formed here depending on water vapor partial pressure. Also, depending on temperature level, a so-called anhydrite modification can additionally be formed. In order to achieve high quality in the case of the calcined gypsum, the temperature profile and the dewatering profile are to be controlled carefully. There must be no overheating, and furthermore, as far as possible, also no inhomogeneities with respect to action of water vapor or temperature should occur. Otherwise, undesired phase conversions can result locally, which would entail a reduction in quality. Direct or indirect methods, in particular, have practical significance for the production of the hemihydrate. In this case, “direct” is understood to mean that the hot gas used for the calcining is in direct contact with the dihydrate. By contrast, “indirect” is understood to mean the use of a heat exchanger for the calcining.
A method for calcining by means of a two-stage process is known from WO 2009/135688 A1. The first stage involves so-called flash calcination, by way of which calcination to the desired residual water of crystallization value of the end product is performed within a very short time (of only a few seconds). Provided in a downstream second stage is a post-reactor, which completes the calcination. Here, the post-reactor has no dedicated heating, but is of thermally passive design and merely has a supply of (hot) wet gas. This method leads to gypsum of fairly high quality, but has the disadvantage that the supply of hot, wet gas can be fairly cumbersome. On the one hand, a relatively large amount of energy is required for this purpose, and, on the other hand, the method requires high regulation quality in order to produce the desired high quality.